Traditional surveying involves two operators working with a theodolite and range pole, or a more complex optical electronic "total station". One operator generally positions the theodolite over a known control point while the other holds the range pole at a series of known or unknown points whose positions are to be checked or measured. A prism mounted on the range pole is sighted through the theodolite and accurate angular and distance measurements to the prism are taken at each point. The positions of the points can then be determined by trigonometry.
An approximately analogous process takes place in modern satellite based surveying. Current techniques involve a reference or base antenna/receiver located over a known point and a single operator who moves about with a roving antenna/receiver or "GPS total station". The operator stops on various generally unknown points to record position information in a data collector using signals transmitted by a minimum number of satellite sources which are above the horizon. Correction data is transmitted from the base site through a telemetry system. The roving antenna is also carried on a range pole which is held by the operator, although the antenna need not be within sight of the reference antenna. A vector or base line is determined from the reference site to the rover.
In real time techniques an actual position is determined and recorded at each point during a survey. Other techniques require post-processing in which data from both the reference and roving receivers is recorded for analysis and determination of actual position coordinates later. Most techniques are also either differential or kinematic. In kinematic surveying at least four satellites must be in view of each antenna at all times and centimetre level accuracy can currently be obtained. Five satellites are required for initialization. Differential surveying allows satellites to be temporarily blocked by obstructions between measurement points, and can provide submeter accuracy, which is sufficient for many purposes. In both kinds of technique actual positions are calculated as latitude, longitude and height with reference to the global ellipsoid WGS-84 or an alternative datum. Local northing, easting and elevation coordinates can then be determined by applying an appropriate datum transformation and map projection.
The satellite positioning system most commonly in use today is the Global Positioning System (GPS) although others such as the Global Orbiting Navigation System (GLONASS) are also in use or under development. Some land based systems which simulate satellite systems over a small area are also being developed to use non satellite signal sources. GPS is based on a constellation of 24 satellites operated by the US Department of Defense. The satellite positions are monitored closely from earth and act as reference points from which an antenna/receiver in the field is able to determine position information. By measuring the travel time of signals transmitted from a number of satellites, the receiver is able to determine corresponding distances from the satellites to the antenna phase center, and then the position of the antenna by trilateration. At present the information content of the satellite signals is deliberately downgraded for civilian users and hence the need to use a reference station for accurate work as mentioned above.
Surveyors and other operators carrying out survey related work use a range of equipment and procedures as will be described further below. A surveyor in the field typically carries a survey control device which provides a portable computer interface to the antenna/receiver. He or she generally navigates around a site setting out or checking the layout of survey points, and recording attribute information for existing features, using the control device as required. The device typically contains a database of points on the site, recorded or estimated during earlier work, and offers a variety of software functions which assist in the survey procedures. The operator is able to input information and commands through a keypad on the device, and view position coordinate data, and numerical or graphical results of the software calculations on a small display. For example, when staking out an item such as a line, arc, slope or surface on the site, the item is defined using existing points, a design point is specified as required, and the surveyor navigates to the point under guidance by the control device. A stake is placed in the ground as closely as possible to the point and the position of the stake is accurately measured using the range pole.
Under other circumstances an operator carrying out survey related work may be involved on a construction site, such as a building or roading project, setting out or checking survey points and design features as work progresses. For example, the operator may be a surveyor or engineer who guides construction workers to ensure that a design is completed according to plan. On other sites workers such as machine operators may be acting independently of a surveyor, following a simple plan based on survey work carried out at an earlier date. For example, a worker operating an excavator removes earth from a ditch in order to lay or repair a utility conduit along a surveyed path. Another worker operating pile driving equipment places piles to create foundations for a building or wharf according to a grid of surveyed or calculated locations. In each case described above, the surveyor, engineer, or machine operator makes use of survey information and visual observations of a physical environment while pursuing their work procedures. Such individuals carrying out a wide variety of survey related activities of this kind would benefit from equipment which provides one or more augmented vision capabilities.
Augmented vision systems that are currently available make use of head mounted display devices to superimpose virtual objects or information on an operator's field of view. Images of the objects are generated by a computer processor which is carried as part of the equipment, and are presented on a see through display in front of the operator's eyes. An image is usually calculated for each eye, as if the object was located at a plane several meters in front of the operator. Augmented vision systems have been under development for several years and a range of equipment is available for limited purposes. One example is the Personal Visual Display System described in WO 95/21395.
A determination of operator eye positions is required in order to calculate and superimpose a virtual object on a real world object in the operator's field of view. Sufficiently accurate positions can generally be determined by combining measurements of the operator head position and head orientation with knowledge of the head dimensions. Studies to date have measured head position using local positioning systems.